Research published last week in Nature appears to add another point to the controversial argument that viruses might be living creatures. Scientists at Tufts University School of Medicine have found a virus that captures its host’s immune system and uses it to destroy the host.
In order to be deemed a real “living creature”, an organism needs to have a genome, be able to grow and make more of itself by replication, and be capable of evolving, or adapting to its environment over successive generations. Humans are living creatures, as we exhibit all of those capacities.
For a long time, scientists have deemed viruses, which are little packages of infective material that can only replicate inside living organisms, to be primitive particles of DNA and RNA, termed a “biological entity.” Thus, not living.
An adaptive immune system is something that an actual living creature contains: over successive replications, immune cells are able to develop mounted attacks against new offenders in the body, and they can pass the memory of these defenses to successive cell generations.
Bacteria exhibit this immune memory with something called a CRISPR/Cas, which is a mini-library (CRISPR) and accompanying set of genes (Cas) that captures segments of DNA from invading viruses, stores them within the mini-library, and uses them as a blueprint for immune surveillance and attack against the virus in future encounters. It essentially learns the virus's deepest secrets---encoded in the virus's DNA---and uses them against it, in genomic terms. And it can pass this knowledge to future generations of bacteria. This is not something we previously considered a capacity of viruses.
Scientists have known for almost a century that there are viruses that infect the bacteria that causes cholera, called Vibrio cholerae, and early reports suggest such viruses can stop a cholera outbreak by destroying the bacteria. Viruses that infect bacteria are called bacteriophages (or phages for short). They have a distinct lunar-lander-esque design, and they deliver their cargo to the insides of bacteria through injection.
Curious about how the cholera-infecting phages work, the Tufts team set out to study one family of these phages called ICP1. First author Kimberley Seed analyzed DNA sequences of phages taken from stool stamples from Bangladesh cholera patients over a decade.
What she found was unprecedented: inside the phage was a complete set of bacterial CRISPR/Cas genes. This is remarkable because it indicates that there is transfer of large chunks of DNA happening between bacteria and phages, even a swap as complex as transferring an entire immune system. It is believed that ICP1 phage acquired the CRISPR/Cas genes at some point more than a decade ago, perhaps ages ago, while infecting a cholera strain that contained the CRISPR/Cas system (curiously, they no longer do).
Intriguingly, there were segments of DNA from the cholera bacteria---its deepest secrets---stored within the mini-library of the phage CRISPR/Cas. This suggested that the phage uses a blueprint of the host’s immune system against the host cholera bacteria.
To verify this, Seed infected bacteria with modified viruses that didn't contain the stolen immune system. Unarmed, the viruses were mostly destroyed. But over time, some viruses managed to tweak the DNA of their CRISPR genes and then go on to shut down their host's defenses once again. These "adapted" phages were able to kill the cholera bacteria with ease.
Scientists hope to one day develop phage treatments to fight superbugs, the antibiotic resistant bacteria responsible for deadly bacterial outbreaks in recent years. Phages might be able to stay a step ahead of bacterial evolution since, at least in this case, they could quickly adapt to new bacterial threats. Their ability to outdo bacteria in an evolutionary arms race may make them better suited to fight bacteria than antibiotics in cases of resistance.
Thus, although viruses are still on the fence between being a “biological entity” and an “actual living creature," having an adaptive immune system that can evolve in successive generations is certainly a point for the “living creature” theory. It doesn't have the genome to produce its own immune system, but it certainly raises the question: Is stealing an immune system just as biologically impressive?
Thanks to Carl Zimmer and Andrew Camilli for their feedback and insights.
